Spacecraft attitude and orbit control are essential requirements for the successful deployment of a space based communication system. Orbit control or north-south station-keeping requires relatively large and infrequent corrective maneuvers. These maneuvers are normally accomplished by the use of gas powered thrusters. On the other hand, the attitude of a spacecraft or satellite is frequently adjusted slightly due to a variety of disturbing torques or forces that alter the spacecraft's position and orientation beyond acceptable limits. These attitude disturbing torques are due primarily to environmental disturbances caused by solar pressure on the surface of the spacecraft, geometric asymmetry of the spacecraft and solar, lunar and earth gravitational and magnetic variations.
To compensate for environmental disturbance torques on orbiting spacecraft, several types of active attitude control systems have been developed. One such system uses the gas powered thrusters, that are primarily intended for station keeping, to correct for attitude disturbances. This is costly, makes only coarse attitude adjustments, is inconvenient and requires the additional mass of propellants for controlling spacecraft attitude.
Another system, known as solar sailing, uses solar radiation pressure acting on specific surfaces on the spacecraft that can be deployed and oriented by actuators. This technique adds weight to the system and reduces system reliability due to the addition of deployable control surfaces and their actuators.
A similar solar sailing system employs solar radiation pressure acting on the spacecraft solar panel surfaces that are oriented by the solar panel drive motors. Control moments generated by solar sailing are obtained by changing the orientation of at least one solar panel. Such a method produces an undesirable windmill torque which is typically compensated for by the use of an on-board momentum wheel which adds weight to the system. The windmill torque on these and similar systems leads to only coarse attitude adjustment. Moreover, when a solar panel is oriented for solar sailing it does not directly track the sun, resulting in a reduction in electrical power.
Another system for achieving attitude control uses on-board magnetically induced fields to react against the magnetic field of the earth or some other celestial body which the spacecraft might be orbiting. One such system entails the use of magnetic torque rods (current carrying coils wrapped around a ferromagnetic core) located inside the spacecraft to generate attitude adjusting torques. This system not only increases spacecraft weight but also unnecessarily consumes spacecraft electrical power. This power might otherwise be used to operate spacecraft payload components. Payload components are generally considered to be those components which execute the spacecraft function. For a communication satellite, payload components are communication equipment, including antennas, amplifiers and antenna directional equipment.
A similar magnetic torquing system uses magnetic torquing coils mounted on the solar wing or panel to generate attitude adjustment torques. This system increases spacecraft weight and requires solar energy to generate the electrical power for attitude control. Unfortunately, this solar energy may be occasionally unavailable. Moreover the torque produced by such a system is marginal due to the small amounts of current flowing through individual solar panel circuits.
Thus, the problem of attitude control of a spacecraft has not been completely solved Prior art attitude control systems increase spacecraft weight or require additional electrical battery power and yield only coarse attitude adjustment. Increasing spacecraft weight and/or electrical power requirements is undesirable because it reduces the useful life of a spacecraft and increases costs. Coarse attitude adjustments over undesirably large increments are not suitable for communication networks that use accurate attitude control to minimize the power required to maintain communication links. Such coarse attitude adjustments lead to misalignment of antennas.